1. Field of the Invention
This invention relates to a control device for internal combustion engine and particularly to a new technique for securely detecting degradation of a ternary catalyst for exhaust gas purification.
2. Description of the Related Art
Generally, in an internal combustion engine, a ternary catalyst containing noble metal such as platinum, rhodium or palladium is used; and at the same time, harmful gas (HC, CO and NOx) contained in the exhaust gas is purified to harmless gas by this ternary catalyst. The purification effect of the catalyst is maintained at a high level near the stoichiometric ratio but it is significantly lowered away from the stoichiometric ratio.
The ternary catalyst contains auxiliary catalysts such as alumina and ceria in addition to the noble metal. These auxiliary catalysts function to prevent thermal degradation of the noble metal. Of the auxiliary catalysts, ceria has such an oxygen occlusion capability as to occlude oxygen when the air-fuel ratio is leaner than the stoichiometric ratio and to release oxygen when the air-fuel ratio is rich than the stoichiometric ratio. It can maintain a constant stoichiometric ratio even when the air-fuel ratio in the ternary catalyst varies. Therefore, even when the air-fuel ratio upstream from the catalyst is varied by acceleration or deceleration, the oxygen occlusion capability of ceria keeps the stoichiometric ratio in the catalyst and thus maintains high purification capability of the catalyst.
FIG. 13 is an explanatory view showing the results of installing air-fuel ratio sensors upstream and downstream from the ternary catalyst and actually measuring the air-fuel ratio downstream from a ternary catalyst in the case where the air-fuel ratio upstream from the ternary catalyst is varied. Case A shows the air-fuel ratio measured downstream from a new ternary catalyst. Case B shows the air-fuel ratio measured downstream from a ternary catalyst that is thermally degraded by accidental fire or the like in the engine. As is clear from Case A shown in FIG. 13, when a new ternary catalyst is used, even if the upstream air-fuel ratio is varied, the stoichiometric ratio is maintained in the ternary catalyst by the oxygen occlusion capability of ceria and the air-fuel ratio downstream from the ternary catalyst is substantially constant without varying.
However, if an accidental fire or the like occurs in the engine and thermally deteriorates the ternary catalyst, the oxygen occlusion capability of ceria is lowered, deteriorating the catalyst purification capability. Therefore, as in Case B shown in FIG. 13, when the upstream air-fuel ratio is varied, the air-fuel ratio downstream from the ternary catalyst varies in accordance with the variation of the air-fuel ratio upstream from the ternary catalyst.
FIG. 14 is an explanatory view showing the relation between the oxygen occlusion capability, and the quantity of non-methane hydrocarbon NMHC (g/mile) and the quantity of nitrogen oxide NOx (g/mile) as the quantities of exhaust gas for the traveling in the United States emission control mode, in the case where the ternary catalyst is thermally degraded by forced accidental fire. As is clear from FIG. 14, as the thermal degradation of the ternary catalyst becomes serious, the oxygen occlusion capability is reduced. Along with the reduction in the oxygen occlusion capability, the quantity of exhaust gas increases and deteriorates. Therefore, if the air-fuel ratio upstream from the degraded ternary catalyst is varied, the air-fuel ratio downstream from the catalyst varies in accordance with the variation of the air-fuel ratio upstream from the ternary catalyst, as in Case B shown in FIG. 13. This means that ceria is degraded and therefore cannot absorb oxygen changes upstream from the ternary catalyst because its oxygen occlusion capability is lowered.
In Europe and the United States, on-board diagnosis control (OBD control) has been enforced to detect deterioration of the performance of the emission system. The OBD control includes detection of degradation of the ternary catalyst as described above. For example, if the ternary catalyst is so degraded that the quantity of exhaust gas exceeds the OBD control level as shown in FIG. 14, a malfunction indicator light (MIL) must be turned on to notify the driver of the malfunction.
Thus, conventionally, a device has been disclosed in which a linear air-fuel ratio sensor that can linearly detect the air-fuel ratio is provided upstream from the ternary catalyst, whereas a rear λ sensor having its output largely changed near the stoichiometric ratio is provided downstream, and in which the air-fuel ratio upstream from the ternary catalyst is varied and a predetermined quantity of oxygen change is given to the ternary catalyst to diagnose degradation of the catalyst from the behavior of the downstream rear λ sensor (see, for example, Patent Reference 1).
With this conventional device, the quantity of oxygen change is set to the oxygen occlusion capability of the OBD detection level (for example, value (A) in FIG. 14). If the oxygen occlusion capability is higher than the quantity of oxygen change, the oxygen change is absorbed and the output of the rear λ sensor is stable. However, if the ternary catalyst is degraded and the oxygen occlusion capability becomes lower than the quantity of oxygen change, the oxygen change cannot be sufficiently absorbed and the output of the rear λ sensor largely varies. Thus, the degradation of the catalyst can be accurately detected.
Patent Reference 1: JP-A-11-270332 (FIGS. 1 to 4, Pages 2 to 6)
However, the air-fuel ratio control system generally performs double-feedback control, that is, feedback control of a target-upstream air-fuel ratio by using the rear λ sensor so as to achieve a target downstream air-fuel ratio, and feedback control of the quantity of fuel injection by using the linear air-fuel sensor so as to achieve a target upstream air-fuel ratio. Therefore, if the above-described conventional technique for diagnosing the degradation of the ternary catalyst is applied, the two feedback controls interfere with each other to cause hunting of the air-fuel ratios upstream and downstream from the ternary catalyst, thus deteriorating emission (particularly NOx) and drivability, lowering the accuracy of the diagnosis of the catalyst degradation, and causing misjudgment, as shown in FIG. 15.
Also, as shown in FIG. 12, if the air-fuel ratio is varied while the central value of air-fuel ratio variation (basic target A/F) remains deviated from the stoichiometric ratio, the oxygen occlusion capability E is saturated and the rear λ sensor output F does not vary, or conversely, the oxygen occlusion capability E is reduced to zero and the rear λ sensor output F does not vary (not shown). Thus, there is a problem that accurate diagnosis of the catalyst degradation cannot be made.
Moreover, in the conventional device, since the air-fuel ratio is varied in step waveforms, the air-fuel ratio changes largely and too acutely, as shown in FIG. 11. Therefore, the air-fuel ratio is not converged to a target value, causing hunting and deteriorating drivability.